It is desirable to find bronchodilating agents which have longer duration of activity than the substances which are available on the market. The compound known under the generic name terbutaline, of the structural formula ##STR6## is one of the presently preferred long-acting bronchodilating drugs on the market and is described i.a. in the U.S. Pat. No. 4,011,258, has a duration of therapeutic activity of about 6 to 8 hours. This duration is confirmed by many years of clinical experience and can be quantified by the finding that a serum concentration of at least about 2 ng/ml of terbutaline is necessary for obtaining the desired therapeutic activity (Hornblad et al., Europ J. clin Pharmacol. 10 9-18 (1976)).
Another long-acting bronchospasmolytically effective compound available on the market, salbutamol of the structural formula ##STR7## has a duration of bronchospasmolytic activity which is about equal to the duration of terbutaline.
Attempts to obtain bronchospasmolytically active compounds with long duration of activity are reported in the literature. Thus, Zolss, Sci. Pharm. 32 (1964) 2 76-92 discloses i.a. certain esters of ethanol amine derivatives known at that time. Minatoya, The Journal of Pharmacology and Experimental Therapeutics Vol. 206 No. 3, 515-527, discusses the pharmacological properties of a compound known as bitolterol of the formula ##STR8## The compound bitolterol which also is disclosed in the Belgian patent 748 178 proved to have a duration of activity comparable with that of salbutamol.
The problem underlying the present invention was to find orally active bronchospasmolytic agents which have a clinically useful duration of activity of at least 12 hours.